Galvanic anode and method of treating the same



United States Patent 3,227,644 GALVANKI ANODE AND METHOD OF TREATING THE SAME Herbert Q. Ruiemiller, Cleveland, Ohio, assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania N0 Drawing. Filed Oct. 5, 1961, Ser. No. 143,042 Claims. (Cl. 204-197) This invention relates to aluminous metal galvanic anodes for the cathodic protection of metals and a process for treating the same to improve their performance and relates particularly to consumable anodes of an aluminum base alloy for the cathodic protection of non-aluminous metal structures exposed to the corrosive action of aqueous media and particularly aqueous saline media, and a process for improving the output of such anodes.

Cathodic protection systems are well known in which a metal article immersed in an electrolyte is protected from corrosion by means of a sacrificial or consumable anode which is also immersed in the electrolyte and is electrically connected to the metal structure (cathode) which is to be protected. Protection against corrosion is particularly important when the metal article is exposed to the corrosive action of an aqueous saline media. Sacrificial anodes are employed to provide cathodic protection for such structures as steel pipe lines, ship hulls, ship ballast tanks, metal sea walls, and drilling rigs.

sacrificial or consumable anodes are generally made in any desired shape or size to suit the structure to be protected and must be composed of a metal which is anodic to the metal body to be protected. The anodes may be in wrought or cast form but the latter has generally been preferred. Some convenient means for attaching the anode to the article to be protected is usually necessary such as an embedded metal core strap, rod or cable.

For many applications the expense of replacing eX- hausted anodes represents a substantial part of the cost of the protective system. For this reason it has been recognized that a long life accompanied by adequate current output is highly desirable for reducing the cost of cathodic protection. This characteristic is referred to as high current efficiency and is generally expressed in terms of ampere hours delivered to the cathode per pound of anode metal consumed. The difference in potential between the anode and cathode must be great enough, of course, to maintain a flow of current. On the other hand, however, too great a difference in potential will shorten the life of the anode without corresponding improvement in cathodic protection.

The object of this invention is to provide an improved aluminum base alloy galvanic anode which has a longer useful life per unit Weight of anode material, with attendant cost savings, than heretofore attainable with the same alloy. Another object is to provide an aluminum base alloy galvanic anode having both a high current efficiency and a substantially constant low electrode potential during its life to continuously protect the metal structures with which it is connected. Still another object is to provide a thermal treatment of the anode which improves its curent efficiency.

I have found that greatly improved cathodic protection is achieved by employing aluminum-zinc type alloy anodes which have been solution heat treated and thereafter rapidly cooled to retain the dissolved alloy constituents in solid solution. The alloy used for such anodes should consist essentially of aluminum, from 3.5% to 9.0% by weight of Zinc and at least one element selected from the group composed of 0.05 to 0.20% by weight of tin and 0.008% to 0.05% by Weight of indium. To obtain the best results, I prefer to use from 6.0% to 8.0% by weight of zinc. All impurities in the aluminum base alloy, such 3,227,344 Patented Jan. 4, 1966 ice as for example iron, silicon and copper, should not exceed a total of 0.50% and more specifically the alloy should contain less than 0.20% iron, 0.20% silicon and 0.02% copper since in greater amounts they reduce the current efficiency of the treated anodes. All other impurities should not be over 0.05% each.

The zinc component of the alloy is necessary to provide the desired electrode potential for the anode. Smaller amounts than 3.5% do not supply the desired characteristics in the anode while more than 9.0% does not produce any added improvement in performance. The elements tin and indium also affect the behavior of the treated anodes. It has been found that they serve to maintain a high current output over the life of the anode. Smaller amounts than the stated minimums have an insignificant effect whereas larger quantities have an adverse effect.

The solution heat treatment of the alloy required to establish the desired condition for high anode performance consists of heating the anodes to a temperature between 800 F. and 925 F., and holding Within this range for a sufficient length of time to effect substantially complete solution of the soluble alloying elements and thereby establish a homogeneous structure. Generally the period of soaking within the foregoing temperature range should extend over a period of from 1 to 12 hours, the length of time being dependent upon the temperature and mass of the anodes being treated. Holding the anodes within the aforementioned tern erature for about 2 hours has been found in many instances to be sufficient and can be considered to be a practical minimum for commercial heat treatment. Heating to a temperature in the lower portion .of the temperature range usually requires a longer time to bring about a solution of the soluble elements than heating within the upper portion of the temperature range. Once the alloying elements are in substantially complete solution and a homogeneous condition is created there does not appear to be any advantage to continue the thermal treatment.

After the anodes have been held at the elevated temperature for a sufficient length of time, they should be rapidly cooled to room temperature. This can be accomplished in a known manner as by quenching in an air blast, by water spray, by immersion in a water bath, or by other means. The particular cooling means employed will in general be determined by the facilities at hand. In order to reduce warpage of the anodes, I have found a quench in hot Water at a temperature of about 180 to 212 F. to be quite satisfactory. No further thermal treatment is necessary or desirable after the drastic cooling operation.

The anodes can be made in either cast or wrought form but gene ally it is most convenient to produce them in the form of castings since the supporting rod or cable can be cast in place. The sand or permanent mold casting procedures are generally most convenient to employ.

The size and shape of the anodes will vary with the type of installation, and generally weigh between 1.0 and 50 pounds.

The treated alloy anodes described above are capable of yielding well over 1000 ampere hours per pound at a substantially constant potential where-as anodes of the same alloy without the thermal treatment produce less than 700 ampere hours per pound. A difference of 0.2 to 0.4 volt in electrode potential is maintained between the treated aluminum alloy anode and the steel structure and thus affords adequate protection on the one hand while on the other hand avoids what is known as over protection. The treated anodes maintain a substantially constant difference in potential during the life of the anodes.

The improvement in anode efliciency resulting from the solution heat treatment is illustrated in the following two examples:

Example 1 Sample anodes of an aluminum base alloy consisting essentially of aluminum, 7.0% zinc and 0.12% tin, with .an impurity content of 0.01% copper, 0.12% iron, and 0.10% silicon, were cast in the form of cylinders in a permanent mold. Half of the lot of anodes was tested in the as-cast condition while the other half was given a solution heat treatment consisting of heating for 2 hours at 850 F. followed by a quench in boiling water before being tested. Each of the as-cast anodes and the solution heat treated anodes were weighed and immersed in synthetic sea water in separate steel drums whose interior surface has been sand blasted prior to the test to remove all rust and scale. The anodes were electrically connected with the drums through a voltmeter having 0.05 ohm resistance to measure the voltage drop and thus determine the current output. Readings were made at regular intervals during the test period of 32 days at the end of which time the anodes were removed, cleaned and weighed to determine the loss of metal. It was found that both sets of anodes had lost about 60% of their weight. The current output, as determined from the readings, showed that the as-cast anodes had delivered 640 to 680 ampere hours per pound of anode consumed, the average being 660, whereas under the same conditions the solution heat treated anodes delivered 1050 to 1170 ampere hours per pound of anode consumed or an average of 1110 ampere hours per pound of anode. Moreover, it was observed that initially the current output from the as-cast anodes was 184 milliamperes and from the solution heat treated anodes it was 182 milliamperes. However, at the end of the 32 day test period the ascast anodes showed a current output of only 61 milliamperes, or a 67% decrease, while the solution heat treated anodes delivered 149 milliamperes or a decrease of only 17%. Only the solution heat treated anodes, therefore, maintained a substantially constant high value during the entire testing period. It was also observed that the corrosion product adhered to the as-cast anodes whereas this did not occur on the solution heat treated anodes. Presumably, the massive coating of corrosion product on the .as-cast anodes interfered with the flow of current and made the surface more resistant to the flow of the electric current than on the uncoated anodes and reduced the effective potential of the anodes.

Example II Sample anodes of aluminum base alloy consisting essentially of aluminum, 7.0% zinc and 0.01% indium, with an impurity content of 0.01% copper, 0.12% iron, and 0.10% silicon, were also cast in cylindrical form in a permanent mold. The lot was divided into two portions as in the preceding example, one was retained in the as-east condition while the other portion was solution heat treated for 2 hours at 850 F. and quenched. The anodes were exposed to the same 32 day corrosion test. Both groups lost 60% of their weight. Those which had been solution heat treated delivered an average of 1100 ampere hours per pound of anode whereas those in the as-cast condition produced only 650 ampere hours per pound of anode. The initial current output of the solution heat treated anodes was 153 milliamperes and the output at the end of the test was 128 milliamperes showing a loss of 16%. The as-cast anodes, on the other hand, had an initial output of 150 milliamperes which declined to 67 milliamperes at the end of the period thus showing a loss of 55%. The thermally treated anodes were substantially free from corrosion product but the as-cast anodes were heavily coated.

In both the above examples the anodes which were solution heat treated had current efiiciencies considerably greater than the as-cast anodes and the current output of the solution heat treated anodes was high and rem'ained high during the entire testing period while the current output of the .as-cast anodes dropped off considerably.

The benefit derived from the presence of a small amount of tin or indium in homogenized aluminum-zinc alloy anodes is illustrated in a comparison with a cast anode of an aluminum-5 .5 zinc alloy of the type heretofore used for cathodic protection purposes. The alloy was cast in a permanent mold and the anodes solution heat treated at the same temperature and under the same conditions as employed in treating the anodes in the preceding examples. The anodes produced an average of only 710 am-peres per pound of metal which is considerably below the yield of the anodes containing tin or indium described above.

Having thus described my invention and certain embodiments thereof I claim:

1. A galvanic anode composed of an aluminum base alloy consisting essentially of aluminum, from 3.5% to 9.0% of zinc and at least one element selected from the group consisting of from 0.05% to 0.20% tin and from 0.008% to 0.05% indium, the total of all impurities not being over 0.50%, said anode having a homogeneous internal structure resulting from a solution heat treatment and rapid cooling and characterized by a higher current efiiciency than the same anode prior to such solution heat treatment.

2.. A galvanic anode in accordance with claim 1, wherein the amount of zinc is from 6.0% to 8.0%.

3. A galvanic anode composed of an aluminum base alloy consisting essentially of aluminum, 3.5% to 9.0% zinc and at least one element selected from the group consisting of from 0.05% to 0 20% tin and 0.008% to 0.05% indium, the alloy containing as impurities up to 0.20% iron, up to 0.20% silicon, up to 0.02% copper and all others not exceeding 0.05% each, the total of all impurities not being over 0.50%, said anode having a homogeneous structure renulting from a solution heat treatment and rapid cooling and characterized by a higher current efliciency than the same anode prior to such solution heat treatment.

4. A galvanic anode according to claim 3 wherein the zinc content is 6.0% to 8.0%.

5. A cast galvanic anode composed of an aluminum base alloy consisting essentially of aluminum, from 3.5% to 9.0% of zinc and at least one element selected from the group consisting of 0.05% to 0.20% tin and from 0.008% to 0.05% indium, the total of all impurities not being over 0.50%, said anode having a homogeneous internal structure resulting from a solution heat treatment and rapid cooling and being characterized by having a higher output of ampere hours per pound of metal consumed than the same anode in the as-cast condition.

6. A cast galvanic anode in accordance with claim 5, wherein the amount of zinc is from 6.0% to 8.0%.

7. A cast galvanic anode com-posed of an aluminum base alloy consisting of aluminum, from 3.5 to 9.0% zinc and 0.05% to 0.20% tin, the alloy containing as impurities up to 0.20% iron, up to 0.20% silicon, up to 0.02% copper and all others not exceeding 0.05% each, the total of all impurities not being over 0.50%, said anode having a homogeneous internal structure resulting from a solution heat treatment and rapid cooling and being characterized by having a higher output of ampere hours per pound of metal consumed than the same anode in the as-cast condition.

8. A method of making a thermally homogenized galvanic anode composed of an aluminum base alloy consisting essentially of aluminum, from 3.5% to 9.0% of zinc and at least one element selected from the group consisting of from 0.05% to 0.20% tin and from 0.008% to 0.05% indium, the total of all impurities not being over 0.50%, said method comprising heating said anode to a temperature between 800 F. and 925 F. for sufficient time to eiiect substantially complete solution of the said alloying elements, and thereafter rapidly cooling said anode.

9. A method of making a thermally homogenized galvanic anode composed of an aluminum base alloy consisting essentially of aluminum, from 3.5% to 9.0% of zinc and at least one element selected from the group consisting of from 0.05% to 0.20% tin and from 0.008% to 0.05% indium, the total of all impurities not being over 0.50%, said method comprising heating said anode to a temperature between 800 F. and 925 F. for 1 to 12 hours, and thereafter rapidly cooling said anode to room temperature.

10. A method of making a thermally homogenized cast galvanic anode composed of an aluminum base alloy consisting essentially of aluminum, from 3.5 to 9.0% of zinc and at least one element selected from the group consisting of from 0.05% to 0.20% tin and from 0.008% to 0.05% indium, the total of all impurities not being over 0.50%, said method comprising heating said anode to a temperature between 800 F. and 925 F. for 1 to 12 hours, and thereafter quenching in hot water at a temperature of about 180 F. to 212 F.

References Cited by the Examiner UNITED STATES PATENTS 1,120,768 12/1914 Uyeno -146 2,075,090 -3/1937 Bonsack et al 75-146 2,076,577 4/1937 Kempf et al. 75-146 2,565,544 8/ 1951 Brown 204-148 2,895,893 7/-1959 Robinson 204-197 2,913,3 84 11/1959 Staley 204-197 2,982,705 5/1961 Sakano 204-197 2,985,530 5/1961 Fetzer et al 75-146 2,993,783 7/1961 Martin 75-146 3,133,839 5/1964 Thomas 75-146 OTHER REFERENCES Aluminum Heat Treating, Renolds Metal Co., 1954, pp. 5969.

JOHN H. MACK, Primary Examiner.

JOSEPH REBOLD, Examiner.

MURRAY TlL-LLMAN, WINSTON A. DOUGLAS, T.

TUNG, Assistant Examiners. 

1. A GALVANIC ANODE COMPOSED OF AN ALUMINUM BASE ALLOY CONSISTING ESSENTIALLY OF ALUMINUM, FROM 3.5% TO 9.0% OF ZINC AND AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF FROM 0.05% TO 0.20% TIN AND FROM 0.008% TO 0.05% INDIUM, THE TOTAL OF ALL IMPURITIES NOT BEING OVER 0.50%, SAID ANODE HAVING A HOMOGENEOUS INTERNAL STRUCTURE RESULTING FROM A SOLUTION HEAT TREATMENT AND RAPID COOLING AND CHARACTERIZED BY A HIGHER CURRENT EFFICIENCY THAN THE SAME ANODE PRIOR TO SUCH SOLUTION HEAT TREATMENT. 